Transmitting system



March 31, 1936. H, CHiRElX 2,035,788

TRANSMITTING SYSTEM Filed Sept. 26, 1931 OSCILLATOR MODULATOR AMPUHER 2 RECTIHER V 6 :12 j j n BR H a 4 O2 L L l as 2 2 5E c: 1 an M INVENTOR HENmcHnzEnk ATTORNEY Patented Mar. 31, 1936 UNHTED STATES PATENT OFFICE pagnie Generale de T France Application September elegraphie, Sans Fil, Paris,

26, 1931, Serial No. 565,215

In France September 27, 1930 4 Claims.

This invention relates to the compensation of low-frequency distortion in radio telephony stations operating with amplification of modulated radio frequency.

Contradistinct to the stations operating on the system known as the plate control or constant current system in which, as indicated by the name, the current derived from the source remains constant during a cycle of audio frequency modulation, stations working on the modulated radio frequency system are characterized by a variable current taken from the source.

Owing to the inevitable presence of the filter the alternating current component of this current is finally furnished by the terminal capacity of the filter. However, even where very high values of this capacity are dealt with, the fluctuations in potential are far from negligible, indeed, they are at all events so much greater the lower the modulation frequency, whence distortion of the low pitches on account of the insufiiciency or inability of the filter tofurnish the instantaneous power.

According to the present invention which is known as the Chireix system a theoretically almost complete compensation is secured in the following fashion: The mean radio frequency excitation of the grids of the tubes of one stage of amplification is caused to vary in accordance with an inverse law of the fluctuations of the potential applied to the last stage.

It will be possible to get an idea respecting the mechanism of such a compensation by the following considerations:

It will be easily understood that the equation for the audio frequency current after rectification is of the following form, to wit:

where E (t) represents the fall of potential (either positive or negative) of the high-potential source, and sin Qt(1+Ef(t)) the audio frequency modulation of the radio frequency amplitude fed or applied to the grids of the last stage.

Now, E by supposition is a rather small number, though not negligible compared with unity, and fit) a function capable of being resolved into a Fourier series having, for instance, 1 as its maximum value.

The second member of Equation (1) demonstrates that the compensation will be perfected to the same degree as E is negligible in reference to unity.

On the other hand, without compensation Equation (1) would be reduced to its first two terms, whence the distortion or deformation referred to at the outset.

All such means as are adapted. to utilize the very fall of potential across the terminals of the filter in order to modulate the audio frequency wave constitutes one of the modes of carrying the invention into effect.

Compensation most preferably is to be effected at a point where the energy level is low, that is to say, for instance, at the very point where the line comes in from the studio. One particularly recommendable arrangement consists in the adoption of a symmetric assembly of tubes (pushpull scheme) for which the line currents excite the grids in opposition or push-pull fashion and the currents due to the fall of potential in addition; the output transformer, on the other hand, being connected to collect the modulation terms.

Fig. 1 attached to this specification by way of example illustrates a scheme embodying the foregoing idea.

Fig. 2 illustrates an alternate coupling arrangement for the output of tubes I-I I.

The line currents coming in from the studio (after optional amplification) come in at I, I and leave at 2, 2', after having passed through the push-pull arrangement indicated by the tubes I and II. Or, more precisely speaking, the input currents traverse the transformer 3 having its secondary shunted by a resistance 4 with the middle point thereof being brought out.

After amplification by tubes I and II the currents fiow across the output transformer 5 whose primary has its middle point brought out, while its secondary is connected upon a resistance 6. The grids of the tubes I and II are excited by means of a supplementary potential taken from potentiometer I. The latter itself is connected with the secondary of a transformer 8 whose primary is connected in parallel relation to a small portion 9 of the voltmeter resistance V which normally indicates the high potential applied to the last stage.

Finally, a polarization or biasing battery I0 connected with the grids of the tubes I and II is adapted to operate the latter inside a certain part of their characteristic which is known to be suitable. The operation is then as follows:

Let in the potential applied to the grid of tube I from the line 121 the potential applied to-the grid of tube I I from the line '02 the potential fed to the grids of tubes I and H from the potentiometer; in other words, '02 is proportional to the rectification fall of potential.

Making the assumption, in line with the usual approximation, that the plate alternating current of the tubes I and II obeys the formula:

The current in resistance 6 will be proportional to the difference, that is v2 evidently has this form void) hence, it will be sufficient to make in order to get:

which as will be seen is really the desired function.

Equation (2) shows, what is one of the characteristic features of the present push-pull scheme, that:

(a) the potential '02 is absent in the output just as what is true of the harmonics or overtones thereof.

(b) although operating inside the knee or curved portions of the characteristics, harmonic 2 of potential '01 is also absent.

Dealing with the problem by supporting that '01 is a composite potential of this form:

it would be found that the amplification is correct; in other words, free from terms standing for doubled incidental frequencies or of terms meaning aggregates or differences of incidental frequencies.

It is to be clearly understood that the invention is susceptible of a great number of modifications which, however, do not affect the spirit underlying the invention. For instance, 'in order to avoid the necessity of the output transformer having to handle the plate potential, the transformer 5 in Fig. 1 could be replaced by the assembly of resistances, capacities and inductances that is indicated in Fig. 2.

Iclaim:

1. A radio transmitter having, in combination, an oscillator, a vacuum tube modulator, and a vacuum tube amplifier, an audio frequency input circuit, a multi-tube amplifier having a pair of tubes which have their corresponding electrodes respectively interconnected, said tubes having grids polarized in such manner that each of the tubes operatesupon the knee of its characteristic, said multi-tube amplifier being connected to said audio frequency circuit and having its output connected to said modulator, a source of po- (a combination sound) tential for supplying voltage to said first vacuum tube amplifier, and means responsive to voltage variations in said source due to instantaneous variations in power of said first amplifier during a modulation cycle, said means being connected between said source and said multi-tube amplifier for changing its negative bias, whereby the audio frequency wave is distorted in a predetermined manner.

.2. A radio transmitting system comprising a plurality of thermionic tube stages in cascade including a high frequency generator and a power amplifier having an output circuit, means for modulating one stage of said cascade arrangement preceding the power amplifier with audio frequencies, a source of potential for applying voltage to the plate of the power amplifier, and compensating means in circuit with said modulating means responsive to voltage variations of said source due to instantaneous variations in power of said amplifier, during the cycle of the audio frequency modulation, for impressing upon the modulated stage a voltage such that the energy in the output of said power amplifier is modulated in conformity with the impressed audio frequency modulation, said modulating means including a pair of vacuum tubes biased so that the current traversing each of them is detected, said tubes being mounted in a push pull arrangement with respect to the modulating means current and in parallel with respect to the compensating means current.

3. A radio transmitting system comprising a plurality of thermionic tube stages in cascade including a power amplifier, a source of potential for applying voltage to the plate of said power amplifier, a pair of push-pull connected tubes having input and output circuits, an audio frequency transformer, the primary winding of which is adapted to be connected to an audio frequency modulatingsource and the secondary winding of which is in said input circuit, said output circuit being coupled to a stage preceding said power amplifier, and circuit means connected to said first source of potential responsive to voltage variations of said first source due to instantaneous variations in power of said power amplifier during the cycle of an audio frequency modulation for supplying a voltage proportional to the fall of potential in said first source to said input circuit in such manner that said potential is applied to said pair of push-pull tubes in parallel.

4. In a radio transmitting system, a generator of local oscillations, a modulator connected to said generator, a power amplifier connected to and amplifying the output of said modulator, a source of low frequency signalling potentials, an amplifier connected to said low frequency source, means connecting said low frequency amplifier to said modulator, a rectifier for supplying plate potential to said power amplifier, the output of said rectifier having an alternating current component, a low frequency alternating current circuit connected to the output of said power amplifier and coupled to said low frequency amplifier for introducing potentials into said modulator for compensating for said alternating current component.

HENRI CHIREIX. 

